In addition to a precious reliquary ampule said to contain blood of the saint, the site holds important works of art, including frescoes by Domenichino and Giovanni Lanfranco in the Royal Chapel of the Treasure of San Gennaro, paintings by Perugino and Luca Giordano in the main church, and fourth-century mosaics decorating the baptistry. To fully grasp the historic and artistic importance of the Duomo di Napoli, visiting with an expert guide is recommended. Underground archaeology walking tours are an excellent way to explore the catacombs, and a combined tour of the most important churches in Naples—Gesu Nuovo, the Sansevero Chapel, and the duomo—covers the highlights of the city in one day.